1. Field of the Invention
The present invention relates to an antenna device, a demodulating device, and a receiving device.
2. Description of the Related Art
When an antenna for receiving broadcasting waves of AM (Amplitude Modulation), FM (frequency Modulation), digital TV, and the like is connected through a feeder cable to a receiving device for processing high frequency signals received by the antenna, the longer the feeder cable is, the more power loss becomes.
In an on-board digital TV receiving system, for example, since a film antenna, which is a transparent film on which a conductor of an antenna element is printed, is attached to a windshield glass or a rear window glass while a receiving device is installed in a center console or the like that is distantly positioned from the windshield glass or the rear window glass, the length of the feeder cable increases.
Thus, in many cases, in order to make up for the power loss by a long feeder cable, such a configuration is employed that an antenna device having a small high-frequency amplifier mounted in a power feeding portion of an antenna is installed and the antenna device is connected through a feeder cable to a demodulating device or the like installed in the center console or the like.
However, since impedance at a power feeding point of the antenna, that is, output impedance of the antenna widely varies depending on received frequencies, impedances of the antenna and of the high-frequency amplifier must be matched.
Such impedance variations become pronounced in the cases of a large ratio bandwidth of broadcasting wave, such as in the case of digital TV broadcasting, the bandwidth of which is 470 to 770 MHz.
However, in order to match impedance, an impedance-matching circuit needs to be included in the antenna device and a control signal for impedance matching needs to be transmitted from the demodulating device to the impedance-matching circuit. This necessitates a control line between the antenna device and the demodulating device in addition to the feeder cable. This results in a higher cost due to increased wiring and involves troublesome installation work.
In order to solve such a problem, Japanese Patent Application Publication No. 4-298122 discloses a vehicle antenna device that includes an antenna provided on a window glass of a vehicle, a dynamic matching circuit connected to the antenna, and a car radio connected to the dynamic matching circuit through a transmission cable, and that can perform impedance matching in a preferable manner.
The dynamic matching circuit includes at least two variable-capacitance diodes and a voltage application circuit for separately applying a voltage to each of the variable-capacitance diodes on the basis of a frequency selection signal of the car radio.
The vehicle antenna device is configured to superpose a received high frequency signal to be transmitted from the dynamic matching circuit to the car radio, the frequency selection signal to be transmitted from the car radio to the dynamic matching circuit, and a supply voltage to be transmitted from the car radio to the dynamic matching circuit onto one transmission cable and then transmit them.
In addition, as a method for encoding a frequency selection signal and transmitting it from a car radio to a dynamic matching circuit, Japanese Patent Application Publication No. 4-298122 discloses two kinds of methods.
A first method is a method for adding a voltage level corresponding to a frequency selection signal to a supply voltage and transmitting them.
FIG. 1 shows a configuration example of a vehicle antenna device based on the first method. A car radio 100 includes a tuner 110, an information converting portion 120 that is tuned to channel selection information (selection frequency) of FM broadcast band at a 1 MHz pitch and outputs a frequency selection signal which is a digital signal, an DA converter 130 for converting the frequency selection signal into an analog signal, a voltage control circuit 150 for generating a frequency signal having the converted analog signal added to the supply voltage to be output from a constant-voltage power supply 140, and a low pass filter 160.
A dynamic matching circuit 200 includes a matching circuit 210 that is placed adjacent to an antenna 300 and performs impedance matching on the basis of a voltage to be applied to variable-capacitance diodes, a high frequency amplifying circuit 290 for amplifying a high frequency signal output from the matching circuit 210, a voltage fixing circuit 230 for reproducing a supply voltage from a frequency signal to be input through a low pass filter 220, a voltage subtracting circuit 240 for subtracting the supply voltage from the frequency signal and retrieving an analog signal, an AD converter 250 for converting the analog signal to a digital signal that is the frequency selection signal, and a voltage applying circuit 260 for separately applying a voltage to each variable-capacitance diode of the matching circuit 210 on the basis of the frequency selection signal.
A second method is a method for transmitting a pulse signal that switches a supply voltage to either high level or low level depending on a pulse train that is generated on the basis of a frequency selection signal.
FIG. 2A shows a configuration example of a vehicle antenna device according to the second method. Description will be made of respects different from the first method. The car radio 100 includes a pulse generating circuit 131 and a voltage control circuit 151 instead of the DA converter 130 and the voltage control circuit 150 in the first method.
The pulse generating circuit 131 generates pulse signals that are binarized on the basis of the frequency selection signals. The voltage control circuit 151 generates a pulse train of 0V or 5V corresponding to the level of the pulse signal (low level or high level).
The dynamic matching circuit 200 includes a supply voltage holding circuit 231, an asynchronous receiving circuit 241, a shift register 270, and a clock generating circuit 280, instead of the voltage fixing circuit 230, the voltage subtracting circuit 240, and the AD converter 250 in the first method.
The supply voltage holding circuit 231 is formed of a high-capacity condenser, a super capacitor, or the like to hold a supply voltage level even when a pulse of low level is input.
Referring to FIG. 2B, the asynchronous receiving circuit 241 recognizes a start bit to be transmitted when a transmission period starts, a data bit having a predetermined number of bits (8 bits in FIG. 2B), an end bit of the transmission period, and a stop bit to be transmitted during a non-transmission period, by synchronizing them to a clock signal to be transmitted from the clock generating circuit 280.
The shift register 270 holds, in a time series manner, a pulse train to be transmitted from the asynchronous receiving circuit 214. The clock generating circuit 280 supplies a clock signal to the asynchronous receiving circuit 241 and the shift register 270.
In both first and second methods, the car radio 100 and the dynamic matching circuit 200 are connected to one another through one transmission cable 400.
In the first method (configuration of FIG. 1) described above, since the dynamic matching circuit 200 must be provided with the voltage fixing circuit 230, the AD converter 250, the voltage applying circuit 260, and the like, the circuit size increases and thus the cost increases.
The second method (configuration of FIG. 2A) described above has three problems to be described below. A first problem is an adverse effect of the clock on the circuit portions constituting the antenna device.
In the second method, the dynamic matching circuit 200 must be provided with the dock generating circuit 280. Generally, since the size of the elements constituting the clock generating circuit 280 of low frequency increases, the clock generating circuit 280 is in many cases configured to use a crystal oscillator or a piezoelectric oscillator of high frequencies (e.g., approximately several tens of megahertz).
The dynamic matching circuit 200 is a circuit that is provided immediately under the antenna 300 and handles high frequency signals. Thus, when the dynamic matching circuit 200 is provided with a clock generating circuit 280 of high frequency, runaround of clock signals and the like may occur, which may cause adverse effects including superposition of noise on a high frequency signal received from the antenna 300.
A second problem is increased circuit size. The voltage control circuit 151 is configured to generate a pulse train only when the received condition of frequency changes while maintaining a voltage of 5V without formation of a pulse train and transmitting the voltage to the dynamic matching circuit 200 when the received condition does not change.
However, it is difficult to implement the asynchronous receiving circuit 241 that recognizes a pulse train, which is transmitted only when the received condition changes, by synchronizing the pulse train with a clock signal, and even if the asynchronous receiving circuit 241 could be implemented, the circuit size would grow and the cost might increase.
A third problem is a need for large-sized components. As described above, the voltage control circuit 151 generates a pulse train of 0V or 5V corresponding to the level of a pulse signal and transmits the pulse train to the dynamic matching circuit 200.
For example, when a communication rate between the car radio 100 and the dynamic matching circuit 200 is 300 bps and an 8-bits pulse signal of low level follows a 1-bit start bit, that is, 9 bits of low level continue, then voltage will not be supplied to the asynchronous receiving circuit 241, the voltage applying circuit 260, and the like for a period of 0.03 seconds on the basis of (Formula 1) shown below:(8+1)/300=0.03   (Formula 1)
However, there must not be any conditions where no voltage is supplied to the asynchronous receiving circuit 241 and the voltage applying circuit 260. Thus, although the supply voltage holding circuit 231 formed of the high-capacity condenser or the super capacitor is necessary, as described above, the element size of the high-capacity condenser or the super capacitor is large in general.